Buoyed by Republican retirements, a significant financial edge over the opposition and their success at elevating children’s health insurance into a top-tier issue, some Democrats have been feeling downright smug, confident of not only holding their House and Senate majorities, but expanding them next year.

But November 2008 is not exactly right around the corner and the man responsible for overseeing the political fortunes of House Democrats is trying to recalibrate expectations. Not his, since Representative Christopher Van Hollen of Maryland says he is well aware of the difficulties ahead. But he wants others to put away any idea of a cakewalk.

“We need to be aware of irrational exuberance and take nothing for granted,” Mr. Van Hollen said. “It is early and a lot can happen in 13 months.”

Democrats have been eager to highlight the plight of Republicans, who have been a cranky minority of late, busy explaining the string of departures, their opposition to the popular health care program and just how they intended to seize back the House on a shoestring budget.

Democrats in the Senate certainly seem to be riding high. Republicans have almost twice the seats to defend. And retirements so far in Colorado, Virginia and New Mexico in particular have created real opportunities for Democratic gains while Republican incumbents in New Hampshire, Minnesota, Maine and Oregon face potentially tough contests.

But the House is another story. As the majority party in an institution where everyone is up for re-election, Democrats automatically have more seats to defend. And as Republicans are fond of noting, 60 Democrats are in districts carried by President Bush the last time around while just 8 Republicans are in districts that went for Democrat John Kerry.

More worrisome to party strategists is the fact that so-called change elections such as 2006 can be followed by elections that change in the other direction. After the Republican revolution of 1994, House Democrats picked up eight seats in 1996, a fact well known to Mr. Van Hollen.

“After a big wave comes in, the wave often subsides,” he said. “We have to beat history to make gains.”

Republicans are also conversant with what might be known as political wave theory. After Democrat Niki Tsongas won a special election in Massachusetts last Tuesday by a 51-45 percent spread, the National Republican Congressional Committee heralded the result with this headline: “The Democratic Wave Breaks.”

Republicans contended that the single-digit victory by the widow of Paul Tsongas in ultra-Democratic Massachusetts showed that Democrats can be had in 2008.

“In a race that should have been won in a walk, Democrats were forced to funnel a massive amount of resources and dispatch an all-star cast of liberal icons at the 11th hour in order to ensure victory,” said Representative Tom Cole of Oklahoma, chairman of the Republican campaign group.

He and party strategists say the respectable showing by Republican Jim Ogonowski demonstrated the appeal of running an anti-Washington campaign at a time when Congress is held in low regard. The Republican view is that Democrats prevailed in 2006 on the premise they would bring change to Washington but voters are not seeing the results.

Republicans might be making more out of the Massachusetts race than the outcome justifies since Ms. Tsongas did actually win and cast her first House vote Thursday. Democrats say her district is not that blue relative to the rest of the state and Ms. Tsongas was not credited with a stellar campaign.

Democrats say the election did wake them up to how Republicans could exploit the anti-illegal immigration theme even in a supposedly liberal region. The push against illegal residents resonates with voters in both parties and could be a major element of the 2008 campaign.

Both parties are also anxiously awaiting the identity of the Democratic presidential nominee. Republicans believe the candidacy of Senator Hillary Rodham Clinton of New York could energize their voters. Some Democrats are worried that her name at the top of the ticket could be problematic for their candidates in more conservative districts though they also believe that a strong presidential contender could help Democrats and hinder Republicans in the Northeast and Midwest in particular.

Democrats have practical reasons for adjusting expectations. They need to keep their activists motivated and, more importantly at the moment, they need to keep those donors worried and writing contribution checks.

But the reality is that the House battle is likely to be tight. House Republicans learned plenty during their years in the majority and they are slowly trying to build a political case against the Democrats.

Mr. Van Hollen still likes his chances.

“We are in a strong position,” he said. “I would rather be in our shoes than theirs right now.”